Altering the pitch of a string on a stringed instrument opens up many avenues of expression for a performer. While most stringed instruments (except pianos and the like) have some means for the performer to alter the pitch by shortening the string length, like pressing a string down behind a fret or placing a bar of metal on the string, these standard techniques come with certain limitations. For example, on a guitar the performer typically only has five fingers of a given length with which to fret notes. This fact inherently limits the number of combinations of chords and melodies that this performer can actuate at a given tempo. Given these physical limitations on chord and melody generation possibilities, there has been much inspiration over the years to develop technologies which allow the performer to alter the pitch of the open string (and therefore all notes fretted on that string as well) in real time. Alteration of the open string pitch enables a performer to sound a different chord or melody without changing the position of the left hand, and therefore increases the avenues of expression available. Furthermore, many styles of music are characterized by the manner in which a performer alters or “bends” certain notes. The “Blues” for example is characterized melodically by frequent upward bending of the minor third.
All such devices which re-tune, bend, or alter the pitch of a string on a stringed instrument are considered to be in the general category of pitch alteration devices. Within this general category, devices are typically developed for either real-time pitch alteration during performance or tuning during the spaces between songs and performances. Re-tuning of a pitch in real-time is not possible in most cases since it takes about 10 to 100 milliseconds for the frequency to stabilize and then another 100 to 500 milliseconds to sample the stabilized frequency and determine what pitch was played (depending on the note played and the tuning algorithm). This required delay for frequency determination is usually longer than acceptable to a performer. Therefore prior art devices typically teach either tuning systems or real-time pitch alteration systems.
Real-time systems require virtually instantaneous tracking of string pitch to a control device which is typically operated by the performer. Examples are pedal steel pitch changers, guitar string benders, tremolo (or whammy) bars, and electronic pitch shifting devices. Tuning systems, on the other hand, do not have to operate in real-time and thus typically require several seconds or longer to move the pitch a small amount. Tuning systems are simply designed to adjust the relative pitch of strings to each other and other instruments so that the resulting music is in tune. Examples are tuning pegs, fine tuners, and automatic tuning devices.
Aside from electronic pitch shifters, which substantially degrade the tonal qualities of the sound, pitch alteration devices of all types are first presented with the problem of the string load. Typical stringed instruments such as guitars tension the strings at about 20 to 40 pounds of tension per string. The prior art teaches ace a number of different machines and methods for reducing this relatively high string load down to a level where a pitch change can be easily actuated by a force supplied by the performer or a motor. It is important to note that the load reduction requirement when the input force is supplied by the muscle power of the human user is substantially less than the reduction requirement if a motor (of reasonable size) is supplying the input force. For example, user actuated lever/spring assemblies on pedal steel guitars typically reduce a 25 pound string load down to about 0.5 pounds which is a total reduction of 50:1. Whereas a motorized pitch alteration device that utilizes modern servo or stepper motors requires a total reduction of 500:1 up to 2000:1, depending on the exact size and torque capabilities of the motor specified. In the discussion that follows I'll use an example of a 2″ thick guitar to illustrate the relationship between the size of various mechanical components and the amount of reduction provided.
To my knowledge, all of the prior art pitch alteration devices utilize at least one or a combination of several of the following machine elements to reduce the string load: (a) lever arm, (b) toggle joint, (c) coiled spring, (d) cam, (e) screw, (f) worm gear, (g) wheel, and (h) various types of standard gearing (planetary gears, helical gears, etc.). In addition to the basic machine elements listed above the following methods are also used to reduce the string load: (a) shunting the bulk of the string load to the body of the instrument and then an actuator bends the string in a direction normal to the axis of the string (i.e. radially) or slides a bridge member beneath the string (therefore actuator does not carry the whole string load); (b) coiling of a string around a post to provide friction for holding the load, and (c) altering the incidence angle of the string force to take advantage of the fact that an oblique force is reduced by the sine of the incidence angle. In the discussion that follows I will refer to the ratio of the original string load to a reduced load as the reduction ratio.
The most common load reduction means is the typical lever as found in tremolo bars, etc. An example is the commercially successful U.S. Pat. No. 4,171,661 to Rose. Within the space constraints of the 2″ thick guitar example mentioned above a lever alone will provide about 2:1 to 20:1 of reduction. The problem is that a lever derives its mechanical advantage by distributing the load out linearly from a pivot. This fact results in a pitch alteration mechanism that is too large or requires too much input force to be practical. The solution as practiced by Rose and most others is to combine the load reduction of the lever with a coiled spring.
While the prior art does teach of camming systems which can partially solve the space problem of levers by wrapping the length required for mechanical advantage around the pivot, all such camming systems either act radially on the string, act on a lever which is connected to the string, or are incorporated into tremolo lever assemblies; see U.S. Pat. No. 2,771,808 to Jenkins, U.S. Pat. No. 5,760,321 to Seabert, and U.S. Pat. No. 6,100,459 to Yost for examples. Since none of these systems are intended to reduce the load, they either exhibit minimal load reduction or limited throw. In theory a typical cam can provide a reduction of 30:1 to 50:1 in the 2″ thick guitar example, but no prior art teaches of such a device. Furthermore, prior art cams provide mechanical advantage by displacing a cam follower which rides on the outside surface of the cam. As the follower gets further from the cam's axis of rotation, the moment arm of the string load increases, partially canceling a portion of the load reduction.
Continuing our guitar example, the common solution of including a coiled extension spring with a lever, as shown by Rose and many others, does increase the reduction ratio up to a range of 20:1 to 40:1. Unfortunately, coiled extension springs of this type also dampen the vibration of the string and increase in force output as the string force decreases (according to Hooke's Law). When there is no motor involved, like in a pedal steel guitar, the increase in force output with decreasing tension is helpful since the spring tends to help return the tension to a zero-point standard pitch without requiring another motion by the user. A motorized system, on the other hand, does not need the assistance of a “return spring”, but would rather include a spring that reduces the load when the string is raised to higher tensions. The prior art teaches at least one example of a lever/spring system for an automatically tuned stringed instrument that optimizes load reduction by careful positioning of the load and spring to take full advantage of the oblique angle force laws mentioned above. However, in addition to the problems mentioned above with this type of system, such a device also doubles the load presented to the lever's pivot when compared to a simple cam and provides a limited range of motion due to the inherent limits of the lever and spring. Furthermore, such devices attempt to match the reduction ratio to the changing string load by matching as close as possible the linear change in spring force due to deflection of the spring to the sum of the sinusoidal changes in spring force and string force due to their respective angle of incidence relative to the common lever. The inherent mathematical difference between the linear spring function and the sinusoidal lever effects means that it is not possible with this method to closely match the reduction ratio to the string load. Further still, these systems are tuning systems and thus cannot provide real-time pitch control; see U.S. Pat. No. 4,909,126 to Skinn et al for an example of an auto-tuning system.
While the prior art does teach a number of load reduction means which utilize lead screws and worm gears, the efficiency of the screw must be set below 50% in order to prevent back-driving (load on the nut or gear drives the screw or worm). This basic fact means that a worm or lead screw on its own is not a good choice for reducing the load down to a level where a small motor can provide the input force. Some additional load reduction is required in motorized applications. For example, the commercially available Robot Guitar, manufactured by Gibson Guitar Corporation, provides a miniature motor which drives a high-reduction gear train which then drives a worm gear. In non-motorized applications the standard worm gear tuning peg is still problematic since it is known to slip slightly and since it requires a coiling of the string around a post. String coiling always results in non-linear stick/slip friction since the coils slide against each other non-uniformly as the tension varies; and this friction results in pitch errors since the non-linear behavior is not easily repeatable. Furthermore, standard tuning pegs do not provide an efficient enough reduction system to allow real-time control of string pitch, except in limited applications.
The use of planetary gears and other types of gear trains are common in the prior art for motorized systems. U.S. Pat. No. 5,886,270 may provide such an example. However, such systems are overly complex, inefficient, expensive, slow, noisy, and high maintenance since the relative inefficiency of the mechanism requires high gear reduction ratios. To my knowledge there is no prior art that teaches of a load reduction system that requires minimal gear reduction prior to the motor.
As mentioned above, another load reduction technique that is employed is to shunt the string load to the body of the instrument and then operate radially on the string, U.S. Pat. No. 4,674,388 to Mathias may provide such an example. While this technique works quite well at reducing the load, it requires too much travel for anything more than small alterations in pitch. Due primarily to the mechanical challenges mentioned above, most real-time pitch alteration devices are non-motorized and are generally operated by the performer's hand, foot, knee, etc. These systems, however, are frequently heavy and complex relative to the number of pitch changes possible. Pedal steel guitars, for example, are quite heavy and can only provide about 20 pre-defined pitch changes at most. Motorized automatic tuning devices are too slow to provide real-time functionality. There are a handful of devices which provide motorized vibrato effects, such as U.S. Pat. No. 4,100,832 to Peterson. But these devices are only capable of minor periodic variations in the pitch of all strings together at the same time. There are very few prior art attempts to my knowledge which provide a motorized real-time pitch alteration system: U.S. Pat. No. 5,038,657 to Busley and U.S. Pat. No. 5,760,321 to Seabert may provide relevant examples.
Some prior art teaches a power-actuated pitch alteration device which includes a foot-operated switch for controlling an electrical solenoid which rotates a cam shaft mounted on the guitar. The solenoid rotates the cam shaft between first and second positions, and tensioning arms engage camming surfaces on the rotating shaft thereby increasing or decreasing the tension in strings attached thereto. The device essentially provides a simplified version of an electrically operated, pedal steel-like pitch changer which does not require the strength of the performer to actuate the bend. Since the force is provided by the closing and opening of a solenoid, the device is fast enough to provide the performer with the ability to change between two different chords in the midst of a performance by simply pressing a pedal. And since the pedal is electrically, not mechanically, linked with the changer on the guitar, performing while standing and moving around is not a problem. While this device does provide a solution to some of the physical limitation issues associated with pedal steel guitars, this device has one major drawback: only two changes are possible. A solenoid is either on or off and therefore it is not possible to get more than two different open string chords with this design. At least one prior art patent teaches a motorized real-time pitch alteration device which includes a string connected directly to a motor shaft. However, there are numerous problems with the design which have prevented this device from ever making it to market. Winding the string around the motor shaft causes improper string return because the string is wound around the motor shaft similar to a typical tuning peg. Improper string return is a well-known problem in the pedal steel art, and it has largely been solved in modern pedal steels by eliminating string terminations which coil the string around a post. One such solution is disclosed in U.S. Pat. No. 4,141,271 to Mullen. The problem arises because as the tension varies, the coils resistively twist around the shank causing a non-linear variable. Improper string return problems are further amplified by the fact that the pitch alteration is provided by coiling a string around a tight radius. This method is not used in any other prior art for real-time pitch alteration because string deformation as the string coils and uncoils around the shaft will cause significant nonlinear errors. Since the motor carries the whole load of the string (up to 50 pounds when raising the pitch) and has to rapidly torque strings up to pitch on raises, it has to be a relatively large motor, which adds bulk and weight to the instrument. A gearhead may be provided as an alternative to help reduce the motor size. However, such a gearhead substantially increases the number of revolutions required to actuate a pitch change, likely slowing the unit down too much to be useful in a real-time system. Such a device also has no physical means to keep the string from loosening, and thus requires the motor to provide the torque required to hold the instrument in tune. This is problematic since there will be lot of heat generated which may damage the wood of the instrument (especially on acoustic guitars), it wastes a lot of power, and it prevents playing of the instrument acoustically since power is required to keep it in tune.
In addition to the above mechanical issues, this attempt does not provide a control system which enables true pedal steel-like pitch changer functionality For example, the device allows any combination of the strings to be pitch altered by any pre-programmed amount, but there is no means provided which allows a user to map multiple control interfaces with a plurality of pitch change operations. Furthermore, there is no control function which provides relative pitch change functionality like a “split tuning” on pedal steel where the actual pitch change is relative to the sum of two pedals. Though this attempt does indicate that the device is capable of correlating frequency of the string with motor location, there is no compensation algorithm given to account for nonlinear variables like those mentioned above plus others that are harder to control like temperature, humidity, instrument deformation, and the like.
Even though tuning systems do not provide real-time pitch control, I have include here some discussion of automatic tuning devices since almost all prior art patents which utilize motors to actuate pitch changes are in this category. A number of inventions have been proposed which seek to automatically tune the pitch of a string or strings via electromechanical means, such as the device to Skinn mentioned above. Such devices include a plurality of motors which are controlled by a computer that “listens” to the frequency of the strings after they have been strummed and then automatically restores to an in-tune state any strings which do not match a pre-determined in-tune pitch. The device is also capable of switching from one predetermined tuning to another. While this may sound at first like similar functionality to a real-time system, all automatic tuning devices that I am aware of are not usable for real-time systems because it takes about 3 seconds to change from one pitch to the next. Furthermore, there is no user interface and control system given which provides real-time access to a plurality of pitch change operations without removing the hands from the instrument, controllable pitch alteration rate, relative pitch function, or pitch change automation. It is not possible for a performer with such a device to change chords along with a tune like a pedal steel player can do, or to strike a first chord, for example, then slowly bend it upwards and have the bending notes reach and stop at a second higher chord right on a specific beat as desired by the performer.
To summarize, the prior art for pitch alteration systems has a number of problems which together have resulted in there being no commercially available device at the current time for providing motorized real-time pitch alteration on stringed instruments. Furthermore, the manually operable real-time systems available are extremely limited in pitch change capability and difficult to actuate due to complex lever systems.
The foregoing patents reflect the current state of the art of which I am aware. Reference to, and discussion of, these patents is intended to aid in discharging my acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated patents disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein.
Furthermore, it is clear from the lack of prior art and the number of problems which still remain unaddressed, that a definite need exists for a real-time pitch alteration device which improves the efficiency of load reduction thereby enabling motorized systems and substantially improving manually-operated systems. And since motorized systems are not very developed at this point in time, there is a further need for a control system which enables a user to accurately actuate a variety of pitch changes in real-time without removing the hands from a playing position, thereby opening up whole new avenues of musical expression which were not previously possible.